1. Field of the Invention
The present invention relates to a cooling structure of a gas turbine combustor tail tube which is cooled by steam as cooling medium.
2. Description of the Prior Art
A combined cycle power plant consists of the combination of a gas turbine plant and a steam turbine plant, wherein a gas turbine takes charge of a high temperature section of thermal energy and a steam turbine takes charge of a low temperature section thereof so as to recover the thermal energy for effective use thereof, and is a power system which is now paid high attention.
In the combined cycle power plant, as a means for enhancing the efficiency, such a cooling system as uses steam from a bottoming cycle, in place of using air, is being disclosed for cooling of a high temperature portion of the gas turbine.
One example thereof will be described with reference to FIG. 2. FIG. 2 shows a prior art cooling structure of a tail tube 1 of a gas turbine combustor. The tail tube 1 is constructed with a tail tube inlet 2 which connects to an upstream combustion chamber (not shown), a tail tube outlet 3 which connects to a downstream turbine (not shown) and a by-pass passage 10, disposed therebetween, which connects to a compressed air chamber (not shown).
The tail tube also includes an inlet side jacket 4 in the vicinity of the tail tube inlet 2, an outlet side jacket 5 in the vicinity of the tail tube outlet 3 and a joining jacket 6 spaced apart from the outlet side jacket 5 by a distance B in the vicinity of the by-pass passage 10.
A cooling steam supply pipe 7 connects to the inlet side jacket 4, a cooling steam supply pipe 8 connects to the outlet side jacket 5 and a cooling steam discharge pipe 9 connects to the joining jacket 6. It is to be noted that, although not shown in the figure, there are provided on an inner wall portion of the tail tube 1 a multiplicity of cooling pipes for leading therethrough a cooling steam from the inlet side jacket 4 and the outlet side jacket 5 to the joining jacket 6.
In the prior art cooling structure of the tail tube constructed as above, the cooling steam supplied from the bottoming cycle enters the inlet side jacket 4 and the outlet side jacket 5, respectively, and then the multiplicity of cooling pipes provided on the inner wall portion of the tail tube 1.
The cooling steam which has entered the inlet side jacket 4 flows in a forwarding flow direction from upstream side to downstream side to cool the inner wall portion of the tail tube 1 and the cooling steam which has entered the outlet side jacket 5 flows in the opposite or adverse flow direction from downstream side to upstream side to cool the inner wall portion of the tail tube 1.
Then, both flows join in the joining jacket 6 to be further sent to downstream side via the cooling steam discharge pipe 9 so as to recover the thermal energy which has been received through the cooling of the inner wall portion.
In the prior art cooling structure of the tail tube as mentioned above, the cooling steam flows in mutually opposing directions, hence if observed separately on the upstream side and on the downstream side of the joining jacket 6, then on the upstream side first, the cooling steam is supplied to a portion where a thermal load is comparatively low to further flow to cool the inner wall of the tail tube 1 with the result that the portion may be cooled excessively sometimes.
On the other hand, on the downstream side, in order to lead combustion gas securely to a gas turbine nozzle, such work of a tail tube outlet 3 portion is needed as to transform a cross sectional shape of the tail tube 1 from an upstream circle shape to a downstream fan shape as the tail tube 1 is contracted toward the tail tube outlet 3 and at same time to bend an axis of the tail tube outlet 3 portion so as to be parallel with a rotor. Thus, the cross sectional area of the tail tube 1 is contracted gradually toward the tail tube outlet 3 so that the flow velocity of the combustion gas is increased resulting in a high thermal load there.
Also, combustion reaction progresses gradually toward the downstream side and influence of secondary flow etc. at the bent portion is added thereto, thereby the thermal load there is further increased. Moreover, the distance B between the outlet side jacket 5 and the-Joining jacket 6, which is approximately equal to that between the inlet side jacket 4 and the joining jacket 6, is considerably long, hence the cooling steam which is introduced into the outlet side jacket 5 and sent to the joining jacket 6 cannot be expected to effect sufficient cooling and the tail tube 1 on the downstream side of the joining jacket 6 is maintained in a state of high thermal load after all.
As the result of the above, there occurs a steep temperature distribution along an axial direction of the tail tube 1 between the upstream side of the joining jacket 6 and the downstream side thereof due to difference in the thermal load, and there is a problem that thermal stress of the tail tube 1 becomes larger.